Plasminogen activator therapy for acute myocardial infarction has resulted in a significant improvement in mortality and morbidity in patients treated early after the onset of symptoms. Despite considerable interest in the development of new, more efficient plasminogen activators, none of those currently available can be administered without risk of hemorrhage. Any improvement that can be made to the specificity or sensitivity of these thrombolytic agents will, in all likelihood, be based on a complete understanding of the structural and functional features of naturally occurring plasminogen activators. The goal of the research proposed here is to gain a precise knowledge of the fibrin binding, inhibitory and catalytic activities of these natural activators. This will be accomplished by generating structurally and functionally intact A (fibrin binding) and B (catalytic) chains from tissue plasminogen activator (tPA) and single- chain urokinase-like plasminogen activator (scuPA), by both chemical and recombinant DNA methods, and then monoclonal antibodies. Of particular interest are the investigations to determine: (i) what precise molecular events result in activation of the catalytic subunit of tPA or scuPA in the presence of fibrin; (ii) the relative contribution of the A chains of tPA and scuPA (despite their structural homology) to fibrin binding, and (iii) whether it is possible to preserve the native function of the A and B chains of tPA and scuPA when fibrin affinity is imparted to them by an antifibrin monoclonal antibody.